How to get the state of a service with sd-bus? - c++

Question

I need to query, monitor and possibly change the state of a few systemd services from a C++ application. It looks like sd-bus is the right way to do this, but I'm having a terrible time finding an example.
So, how do I:
1) Query the current status of a service via sd-bus, similar to systemctl status foo.service?
2) Monitor the status of a service such that I get a callback whenever it changes?
3) Change the status of a service, similar to systemctl start/stop/restart?
Thanks!

Answer 1

Using the sd-bus API is absolutely correct (header #include <systemd/sd-bus.h>)
First you need get access to a bus object:
I do this:
Systemctl::Systemctl() :
m_bus(nullptr)
{
int r = sd_bus_default_system(&m_bus);
if (r < 0)
throw exception("Could not open systemd bus");
}
If you're having problems opening the bus:
Run as root/sudo
Make some polkit policies to grant your user/group access to this command
Run the _user bus instead of the _system bus
Don't forget to release the bus when you are done:
Systemctl::~Systemctl()
{
sd_bus_unref(m_bus);
}
Now you had 3 questions:
Query the status
For each unit, I have a class which holds the escaped name (foo_2eservice) as m_name, and a reference to the bus in m_bus. Call this method with any property. You seem to be most interested in "ActiveState" or "SubState".
std::string Unit::GetPropertyString(const std::string& property) const
{
sd_bus_error err = SD_BUS_ERROR_NULL;
char* msg = nullptr;
int r;
r = sd_bus_get_property_string(m_bus,
"org.freedesktop.systemd1",
("/org/freedesktop/systemd1/unit/" + m_unit).c_str(),
"org.freedesktop.systemd1.Unit",
property.c_str(),
&err,
&msg);
if (r < 0)
{
std::string err_msg(err.message);
sd_bus_error_free(&err);
std::string err_str("Failed to get " + property + " for service "
+ m_name + ". Error: " + err_msg);
throw exception(err_str);
}
sd_bus_error_free(&err);
// Free memory (avoid leaking)
std::string ret(msg);
free (msg);
return ret;
}
Monitor the status of a service:
The first step is to set up a file-descriptor to subscribe to changes. In this case you are interested in subscribing to the "PropertiesChanged" signal. Note that you'll get a signal for any property changing, not just the state. In the sd_bus_add_match() call, there is room for a callback, though I haven't experimented with it.
void Systemctl::SubscribeToUnitChanges(const std::string& escaped_name)
{
/* This function is an easier helper, but it as only introduced in systemd 237
* Stretch is on 232 while buster is on 241 . Need re replace this as long as
* we still support stretch
sd_bus_match_signal(
m_bus,
nullptr, // slot
nullptr, // sender
std::string("/org/freedesktop/systemd1/unit/" + escaped_name).c_str(), // path
"org.freedesktop.DBus.Properties", // interface
"PropertiesChanged", // member
nullptr, // callback
nullptr // userdata
);
*/
std::string match = "type='signal'";
match += ",path='/org/freedesktop/systemd1/unit/" + escaped_name + "'" ;
match += ",interface='org.freedesktop.DBus.Properties'";
match += ",member='PropertiesChanged'";
sd_bus_add_match(
m_bus,
nullptr, // slot
match.c_str(),
nullptr, // callback
nullptr // userdata
);
}
Instead what I do is periodically poll the bus for the subscribed changes and update each unit:
bool Systemctl::ProcessBusChanges()
{
bool changed = false;
sd_bus_message* msg = nullptr;
// for each new message
std::list<std::string> escaped_names;
while( sd_bus_process(m_bus, &msg) )
{
// Note: Once sd_bus_process returns 0, We are supposed to call
// sd_bus_wait, or check for changes on sd_bus_get_fd before calling
// this function again. We're breaking that rule. I don't really know
// the consequences.
if (msg)
{
std::string path = strna( sd_bus_message_get_path(msg) );
sd_bus_message_unref(msg);
std::string escaped_name = path.erase(0, path.find_last_of('/')+1 );
escaped_names.push_back(escaped_name);
changed = true;
}
}
escaped_names.sort();
escaped_names.unique();
for (auto unit : escaped_names)
{
auto it = m_units.find(unit);
if (it != m_units.end())
it->second.RefreshDynamicProperties();
}
return changed;
}
If it tells us that the bus has changed, then I go ahead and read all of my monitored units on that bus.
Change the status
This one is easy. I use the following, where method is one of "StartUnit", "StopUnit", or "RestartUnit".
static void CallMethodSS(sd_bus* bus,
const std::string& name,
const std::string& method)
{
sd_bus_error err = SD_BUS_ERROR_NULL;
sd_bus_message* msg = nullptr;
int r;
r = sd_bus_call_method(bus,
"org.freedesktop.systemd1", /* <service> */
"/org/freedesktop/systemd1", /* <path> */
"org.freedesktop.systemd1.Manager", /* <interface> */
method.c_str(), /* <method> */
&err, /* object to return error in */
&msg, /* return message on success */
"ss", /* <input_signature (string-string)> */
name.c_str(), "replace" ); /* <arguments...> */
if (r < 0)
{
std::string err_str("Could not send " + method +
" command to systemd for service: " + name +
". Error: " + err.message );
sd_bus_error_free(&err);
sd_bus_message_unref(msg);
throw exception(err_str);
}
// Extra stuff that might be useful: display the response...
char* response;
r = sd_bus_message_read(msg, "o", &response);
if (r < 0)
{
LogError("Failed to parse response message: %s\n", strerror(-r) );
}
sd_bus_error_free(&err);
sd_bus_message_unref(msg);
}